Ivermectin liquid vs. Heartgard for heartworm preventative? More economical / cost effective?

I have 6 dogs. When we were still at 3, I saved money by buying the genertic heartguard from Petshed. It's sourced from Australia and for 6 pills + shipping you are paying about $25. When I went to 6, I went to the liquid ivermectin (it's injectable for farm animals, it's ingested for dogs). The rounding of the numbers is fine...we give about .10 cc per dog for HW prevention for labs. You can up it a bit if you want to cover other worms as well. BUT...a 500ml bottle is going to cost you about $50 and they typically only have a shelf life of 1 year. So why not just buy the generic? It will have the same yearly cost and you'll have EXACT dosage for your dog's size without messing around.